This invention relates generally to digital data communications apparatus and more specifically to a device which may be interposed between two data transmitters/receivers and an associated modem so that the modem is effectively shared by the plural data sources. As used heein, the term "data source" may include such things as a digital computer, a line terminal, a terminal control unit or, in fact, any digital device which operates on a request/acknowledge basis with an associated modem. The term "modem" is an acronym for modulator-demodulator and is a device which accepts input binary data signal patterns and produces a resultant data pattern which may be transmitted over a communications link, such as a voice grade transmission line, to a corresponding receiving modem which, in turn, functions to recreate the original data pattern.
A typical system in which the present invention may find application is shown in FIG. 1 of the Jablonski U.S. Pat. No. 3,525,077 assigned to the assignee of the present invention. In this system a plurality of line terminals (LT's) each have associated therewith a modem device. In large systems involving many line terminals, the cost of providing discrete modems for each can be unduly excessive. The present invention allows two LT's to share a common modem and because the cost of the invention is low compared to the cost of typical modems, significant savings can be realized.
Prior art schemes for usng fewer modems than the number of data sources employed in the system have involved complex and therefore costly multiplexing arrangements. Exemplifying such an approach is the invention described in the Norberg U.S. Pat. No. 3,485,953. In this prior art arrangement, means are provided for transmitting data from a central station to a plurality of remote stations via a single communications channel. Each remote station has its own control unit associated with it and each of the control units is intercoupled so that when an identifying code is transmitted, only one remote station control unit can respond to that code to the exclusion of the remaining control units. In the present invention, however, two remote stations (line terminals, computers, etc.) share a single control unit and modem thus reducing the complexity and attendant costs.
The Haglund U.S. Pat. No. 2,131,434 relates broadly to the present invention in that it describes a method of assigning a transmitting channel of a telegraph system jointly to two branch transmitting stations. Means are provided for alternating between the two stations so long as both have messages to transmit and to remain locked to a single station so long as only that station is transmitting. However, the Haglund device is electromechanical in nature and operates in a fashion substantially different from the modem sharing device of the present invention.
In accordance with the teachings of the present invention, a Modem Sharing Device (MSD) is provided to control the flow of data from two terminals on a single line to another station. The MSD includes first and second flip-flops respectively associated with data sources or terminals 1 and 2. An RTS signal from terminal 1 or terminal 2 is ANDed with a signal from the clear side of a flip-flop corresponding to the other terminal to produce an RTS signal which is transmitted to the associated modem. The modem responds to this RTS signal by returning a CTS response signal. However, before the CTS response signal is received from the modem, a positive-going clock signal is used to set the associated flip-flop to which the RTS signal has passed. When the MSD receives the CTS signal from its associated modem, it is allowed to pass only through an AND gate associated with the set one of the two flip-flops. Thus, the CTS from the modem is only returned to one of the two associated terminals, depending upon which one caused the associated flip-flop to be set. One of the two flip-flops in the MSD is clocked by positive-going pulses from an oscillator while the other flip-flop is clocked by its subsequent negative-going edge. As such, a race condition is obviated in that the two flip-flops may not be simultaneously set.